Isotropic high density graphites have been widely used in applications such as electrodes for electric discharge machining, hot press dies, crucibles for aluminum vapor deposition and semiconductor production and various types of heat- and corrosion-resistant tools. The light weight and self-lubricating properties of isotropic high density graphite are the focus of considerable research, and the research and development of this material as a piston component for gasoline engines is being energetically pursued.
The most commonly used materials in manufacturing a piston component for gasoline engine are aluminum alloy. Graphite is a promising substitute for aluminum alloys because it provides practical advantages such as reduction of piston weight, reduction of friction loss owing to the self-lubrication property inherent in graphite, and reduction of gasoline and oil consumption. It also provides environmental improvements such as the reduction of engine noise. Furthermore, the application of graphite to piston components has been proven to reduce both hydrocarbon (HC) content and nitrogen oxide (NOx) content of engine exhaust gas. This particular characteristic of cleaner exhaust gas comes from the effect of better piston shape, which can be realized as a direct result of graphite's lower thermal expansion coefficient compared to those of aluminum alloys.
An aluminum alloy piston usually has a dead space between the piston head and the piston cylinder wall to absorb the thermal expansion effect. A graphite piston, however, owing to its lower thermal expansion coefficient, allows a straight cylinder wall face. Therefore, uneven combustion in the dead space is eliminated with a graphite piston, and efficient combustion is realized. The resulting exhaust gas contains less HC and less NOx.
Although graphite has many advantages as a piston component as described above, it has the disadvantage of insufficient mechanical strength in practical use. A piston component is exposed to repeated jarring impacts caused by explosive combustion. Ordinary graphite does not have enough strength to survive normal conditions of use. For piston components for vehicle gasoline engines, an ultra-high strength graphite is required with a bending strength of at least 120 MPa.
European Patent No. 0258330B1 disclosed a piston comprising a graphite having a minimum bending strength of 75 MPa and covered with a protective layer of silicon carbide or the like and a piston comprising carbon mixed with a ceramic, metallic, or carbon fiber. However, they do not achieve the strength required.
The usual industrial manufacturing process for isotropic graphite comprises a kneading step to mix an aggregate consisting of petroleum or coal coke powder with a pitch binder, a molding step to mold isotropically the raw material powder previously prepared by re-pulverizing the kneaded product of the preceding step using the rubber press [cold isostatic press (CIP)] method, and a carbonization and graphitization step where the molded product is fired to carbonize and graphitize it. In this process, to increase the material density and strength, the addition of the binder pitch component is increased and the degree of contraction of the material during carbonization is increased to enhance the densification of the structure. Increased pitch addition, however, is accompanied by the generation of a large quantity of volatile matter during carbonization, with the disadvantage that defective phenomena tend to occur such as cracks and voids in the material structure. To eliminate this type of problem, several countermeasures have been proposed such as adjusting the pitch composition applied and adjusting the quantity of volatile matter generated after kneading. Other means disclosed in Japanese Unexamined Patent Publication No. 1990-69308 and Japanese Examined Patent Publication No. 1991-69845 propose that raw coke having specified characteristics obtained from a specific manufacturing process be used to produce isotropic carbon material without adding binder.
Regarding the densification of graphite structure, it is reported that the most effective close packing method is to control the particle size distribution in the raw material aggregate coke powder. Nevertheless, this method risks causing fracture of the graphite which results from increased stress intensity appearing particularly around coarse particles. This stress intensification comes from the non-uniform packing structure which consists of a packing of coarse particles, with fine particles filling the interstices. Consequently, an effective means of obtaining an isotropic graphite with a high strength is to use a coke powder which has a uniform fine particle size and wettability with a relatively small amount of pitch binder. However, this type of coke powder has not yet been introduced, and up to now this has created a barrier to manufacturing a high strength, isotropic densified graphite.